Three different methods are primarily employed for flameproofing thermoplastic molding compositions using halogen-containing flame retardants. Firstly, the flame retardants can be admixed with the pulverulent thermoplastics by the plastics manufacturer prior to production of the granulated material, optionally together with synergistic agents. Such a granulated material can then be worked into flame retardant molded components by the plastics processor on conventional extruders or injection-molding machines. Secondly, the processor can perform the task of admixing flame retardants with the synthetic resin granules, using flame retardant concentrates (master batches) in granulated form for this purpose. A third possibility, available to the manufacturer as well as the consumer of synthetic resins, is to mix the powdery flame retardants with synthetic resin granules in a mixer, with the concomitant use of adhesion promoters.
When the first method is employed, the processor lacks the capability of affecting the burning behavior of the supplied molding compositions and of the manufactured products which are subjected to various flame retardance tests. In such a case, the processor must employ the second or third method.
However, difficulties have been found to occur in the mode of operation of the second method, inasmuch as the synthetic resin granules and the granules of the flame retardant concentrates are of differing specific gravity. Consequently, especially when a pneumatic conveyance of the mixtures is involved, non-mixing can very readily occur resulting in inhomogeneous products with nonuniform properties.
Similarly, difficulties are also encountered in the third process. Although the thus-manufactured products can be adapted to the required flame retardance standards very easily by adding varying amounts of flame retardant, this method is disadvantageous since the flame retardant components employed must have melting points which are below the processing temperature of the respective thermoplastics. Higher-melting components are not sufficiently homogeneously distributed during plasticization in the mixing portion of the screws, which portion is short in most cases. As a consequence, the injection-molded or extruded synthetic resin articles have inhomogeneous zones (fisheyes or striae) with substantially modified physical properties. Thus, in such molded components, the tensile strength, the tear strength, and the elongation at rupture is in many instances reduced to 35% and the impact or notch impact strength to 60%. Even if the flame retardant components are ground very finely, this difficulty cannot be overcome in most cases.